Respiratory apparatus commonly have means to alter the humidity of the breathable gas in order to reduce drying of the patient's airway and consequent patient discomfort and associated complications. The use of a humidifier placed between the flow generator and the patient mask, produces humidified gas that minimizes drying of the nasal mucosa and increases patient airway comfort. In addition in cooler climates, warm air applied generally to the face area in and about the mask, as may occur inadvertently by a leak, is more comfortable than cold air.
Many humidifier types are available, although the most convenient form is one that is either integrated with or configured to be coupled to the relevant respiratory apparatus. While passive humidifiers can provide some relief, generally a heated humidifier is required to provide sufficient humidity and temperature to the air so that patient will be comfortable. Humidifiers typically comprise a water tub having a capacity of several hundred milliliters, a heating element for heating the water in the tub, a control to enable the level of humidification to be varied, a gas inlet to receive gas from the flow generator, and a gas outlet adapted to be connected to a patient conduit that delivers the humidified pressurized gas to the patient's mask.
Typically, the heating element is incorporated in a heater plate which sits under, and is in thermal contact with, the water tub.
The humidified air may cool on its path along the conduit from the humidifier to the patient, leading to the phenomenon of “rain-out”, or condensation, forming on the inside of the conduit. To counter this, it is known to additionally heat the gas being supplied to the patient by means of a heated wire circuit inserted into the patient conduit which supplies the humidified gas from the humidifier to the patient's mask. Such a system is illustrated in Mosby's Respiratory Care Equipment (7th edition) at page 97.
Such a heating method for the patient conduit may only provide poor heat transfer due to the wire locating itself along the conduit wall rather than in the main gas stream. A wire will also only give poor turbulent mixing due to its low profile. As a result heat transfer may be poor and the mixing of water vapor and gas may also be poor.
Alternatively the heating wire circuit may be located in the wall of the patient conduit. Such a system is described in U.S. Pat. No. 6,918,389.
U.S. Pat. No. 6,918,389 describes a number of humidifier arrangements for supplying low relative humidity, high temperature humidified gas to the patient. Some of these arrangements include pre- or post-heating of the gas to reduce the relative humidity.
WO 2007/019268 A1 discloses a low cost CPAP flow generator and humidifier assembly, including a heating element, which may be a ribbon heater, placed inside the tub of the humidifier. WO 2007/019628 A1 does not disclose that the heating element may be provided in portions of the assembly other than the humidifier tub, including, for example, the hoses or conduits.
None of these prior art devices provides an entirely satisfactory solution to the provision of comfortable humidified breathable gas to the patient, nor to ease of construction and hygiene requirements and to energy and patient comfort requirements at startup.